The invention relates generally to a method for gas turbine control and more specifically to a method for detecting the onset of liquid fuel in a gas turbine combustor to permit control of the gas turbine during a transfer from operation with a gas fuel to operation with a liquid fuel.
Industrial gas turbines are often capable of alternatively running on liquid and gaseous fuels, e.g., natural gas. These gas turbines have fuel supply systems for both liquid and gas fuels. The gas turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off. Similarly, when the gas turbine burns gaseous fuel, the liquid fuel supply is turned off. Fuel transfers occur during the operation of the gas turbine as the fuel supply is switched from liquid fuel to gaseous fuel, and vice versa.
Gas turbines that burn both liquid and gaseous fuel require a liquid fuel purge system to clear the fuel nozzles in the combustors of liquid fuel. The liquid fuel supply system is generally turned off when a gas turbine operates on gaseous fuel. When the liquid fuel system is turned off, the purge system operates to flush out any remaining liquid fuel from the nozzles of the combustor and provide continuous cooling airflow to the nozzles.
FIG. 1 is a simplified schematic diagram of an exemplary gas turbine having liquid and gas fuel systems. FIG. 1 shows schematically a gas turbine 100 having liquid fuel system 102 and a liquid fuel purge system 104. The gas turbine is also capable of running on a gas, such as natural gas, and includes a gaseous fuel system 106. Other major components of the gas turbine include a main compressor 108, a combustor 110, a turbine 112 and a controller 114. The power output of the gas turbine 112 is a rotating turbine shaft 116, which may be coupled to a generator 130 that produces electric power.
In the exemplary industrial gas turbine shown, the combustor may be an annular array of combustion chambers, i.e., cans 118, each of which has a liquid fuel nozzle 120 and a gas fuel nozzle 122. The combustor may alternatively be an annular chamber. Combustion is initiated within the combustion cans at points slightly downstream of the nozzles. Air from the compressor 108 flows around and through the combustion cans 118 to provide oxygen for combustion. Moreover, water injection nozzles 124 are arranged within the combustor 110 to add energy to the hot combustion gases and to cool the combustion cans 118.
The air for the liquid fuel system purge may be provided from the compressor 108, boosted by a purge air compressor (not shown) and controlled by other elements of the system (not shown). When the gas turbine 100 operates on natural gas (or other gaseous fuel), the liquid fuel purge system 104 blows compressed air into the liquid fuel system 102 through the liquid fuel nozzles 120 of the liquid fuel 102 system to purge liquid fuel and provide a flow of continuous cooling air to the liquid fuel nozzles 120.
FIG. 2 is a simplified diagram of a gas turbine engine with an existing liquid fuel system. Liquid fuel is provided to the liquid fuel system 200 from a liquid fuel source 205. The liquid fuel system 200 includes a flowpath to a flow divider 230 through a low pressure filter 210, a fuel pump 215, a bypass control valve 220, and a stop valve 225. Pressure relief valve 235, bypass control valve 220 and stop valve 225 may recirculate liquid fuel flow through recirculation line 240 to the upstream side of the low pressure filter 210. The flow divider 230 divides liquid fuel flow into a plurality of liquid fuel flow paths leading to individual combustion cans 270. Each liquid fuel flow path downstream of the flow divider includes a 3-way (endcover) valve 245 and a distribution valve 260 before entering the combustion can 270.
Three-way valve 245 permits flow to the combustion can nozzles from the liquid fuel flow path (described above) or from a liquid fuel purge air system 280. Three-way valve 245 is designed to selectably allow flow to the combustor nozzles 120 from the liquid fuel while preventing backflow of fuel to the liquid fuel purge air system or to allow purge air to the combustor nozzles 120 while preventing backflow of purge air into the liquid fuel system upstream of the three-way valve. By preventing purge air from entering the liquid fuel system, the air-fuel interfaces with the fuel supply are minimized.
When gas fuel is supplying the turbine, the 3-way valve 245 is positioned to block liquid fuel flow and allow purge air to pass for cooling the fuel nozzles in the combustor. This purge must be shut off when liquid fuel is turned on. The discussion that follows describes a current generation fuel system, which uses a 3-way (endcover) isolation valve (Traver et al., U.S. Pat. No. 6,438,963) to separate the purge air from the liquid fuel. The invention does not require this hardware; the use of 2 check valves, or 2 way on/off valves could be employed.
During a transfer from one fuel source to another, it is desired that continuity of turbine output power be maintained while minimizing any undershoots or overshoots of output power and temperature. In a transfer from operation with a gas fuel to operation with a liquid fuel, the 3-way valve 245 is switched to the liquid fuel line, the stop valve is opened, and the control valve is commanded to some small “prefill” flow. As the liquid fuel line is prefilled, the gas fuel is held at a required demand reference for the generator output output. The liquid fuel must refill the piping, which was previously filled with purge air, before liquid fuel reaches the combustor nozzles 120.
The piping between the 3-way valve 245 and the combustion can 270 has a known volume that is always filled with purge air before a liquid fuel transfer. The known volume 290 is kept small to minimize the affect of purge introduction (causing a load spike). Leakage of air into the system will occur over time. The rate of air leakage into the system is dependent on a number of variable factors, which cannot be controlled or predicted. This results in the piping upstream of the 3-way valve 245 being filled with some volume of air, dependent mostly on the time since the last fuel transfer. While the physical volume of piping is known, the amount filled with air is unknown. Since the volume of the piping upstream is also much greater than the volume downstream of it, an unknown and potentially substantial volume 295 of air must be displaced with prefill liquid fuel before the liquid fuel reaches the combustors 270.
The foregoing factors may cause poor reliability of gas to liquid fuel transfers, including power overshoots and undershoots and sometimes dropped load. Preventive maintenance procedures for the liquid fuel system require transferring to liquid fuel operation periodically to exercise the system. Difficulty with the transfers may discourage operators from carrying out the transfers necessary for exercising the liquid fuel system, exacerbating the reliability problems of the system.
Historically, liquid fuel prefill has been controlled with only the flow measurement feedback, using an open loop setting (not closed on any global parameter). Upon selection of liquid fuel, the bypass control valve 220 is controlled to a nominal value to reach a target prefill flowrate and held there for a specified period (delay time) before the fuel control is ramped up to the full load reference. The prefill has seen changes over the years to deal with a number of issues in addition to the basic goal of prefilling the fuel system. At one time, the prefill flowrate was set to a very low value (approximately 2%) for an extended period of time, typically 30 seconds. Later, the prefill period was increased to 60 seconds and 120 seconds in some cases. The extension of the prefill period was done to have confidence that the liquid fuel lines would be prefilled, and at the same time not to introduce too much “uncontrolled fuel” through use of the open loop setpoint.
Controlled fuel is fuel that is provided to the combustors in response to a fuel reference demand for a given power output. Uncontrolled fuel is fuel that is introduced to the combustors, but which is not recognized in the turbine control fuel demand signals. Liquid fuel prefill is not included in the fuel demand calculation to avoid a detrimental dip in load if the liquid fuel prefill does not reach the combustors as expected, thus creating an under-fueled condition. The side effects of the uncontrolled fuel reaching the combustors is to supply additional energy resulting in an initial load and temperature spike and then causing global fuel demand to be driven down over time to hold the same load output.
The extended prefill period (60 seconds to 120 seconds) was used to ensure complete prefill of the liquid fuel lines, in the face of uncertainty about the unknown amount of air in the system. The current control is only an estimate to ensure the presence of liquid fuel, because the volume to be filled is an unknown that can vary with many factors.
FIG. 3 illustrates an existing timer based algorithm for control of a transfer from gas fuel to liquid fuel. In step 310 a fuel transfer request is initiated, in which the stop valve in the liquid fuel system is commanded to open to a prefill position. The system waits for the stop valve to start moving in step 320 before a command is issued in step 330 for prefilling to commence. In this existing algorithm, a fixed time delay, shown at an exemplary value of 20 seconds is initiated in step 340. The fixed 20 second time delay is provided to allow the liquid fuel lines to prefill and fuel to reach the nozzles. When the fixed time delay has expired, the fuel transfer between the gas fuel and the liquid fuel is initiated in step 350. After the time delay of step 340 has expired, the liquid fuel prefill (uncontrolled fuel) is ramped off in step 360. Finally, the gas fuel line is purged with air in step 370.
FIG. 4 illustrates, in curve 1, the response of power output (driven megawatts output at the generator) of a gas turbine to an over-prefill of liquid fuel due to the prefill delay period being too long. Initially, gas fuel is being provided to deliver about 58 driven megawatts. At about 18 seconds, a liquid fuel prefill at 2 is commenced. At about 38 seconds, the driven megawatts begins to rise at 3 in response to the prefill liquid fuel (uncontrolled fuel) reaching the combustors, adding to the energy beyond that called for by global demand to maintain the 58 driven megawatts. Starting at about 40 seconds, the global demand for the gas fuel is slightly reduced at 4 due to the incremental energy being supplied by the uncontrolled liquid prefill fuel. At the end of the prefill delay period at 5, the fuel switchover occurs with the gas fuel being supplied ramped off and the controlled liquid fuel being ramped up. The prefill, in this case, extends about 30 seconds at 6 beyond the onset of liquid fuel at 3.
If the delay is set too long, as occurs in FIG. 4, the system will be running with uncontrolled fuel. In addition, for a low prefill value of approximately 1%-2%, the errors in the system would not be a guarantee of repeatable prefills, which drove the longer prefill periods. For emergency transfer conditions where the gas fuel is being lost and switching to liquid fuel must be done quickly, many failed transfers may occur.
FIG. 5 illustrates, in curve 1, response of power output (driven megawatts) of a gas turbine to an under-prefill of liquid fuel due to the prefill delay period being too short. Prefilling begins at 2 (about 41 seconds). When the prefill delay period is set too short, the system will start ramping gas fuel off before the arrival of liquid fuel at the combustors. Initially, gas fuel is being provided to deliver about 25 driven megawatts. At about 44 seconds, a fuel switchover at 3 commences with a ramp-off of the gas fuel and a ramp-on of the liquid fuel supply. The 4 second prefill is not sufficient to fill the liquid fuel line, so actual delivery onset 4 of liquid fuel to the combustors does not begin until about 48 seconds. Between 44 and 48 seconds (at 5) the combustor is undersupplied, leading to a drop-off of driven megawatts to about 21. At 48 seconds with the onset of liquid fuel delivery at 4 to the combustor nozzles at a higher ramped rate, a power spike at 6 to about 33 driven megawatts results.
Under-prefilling has two negative side effects. First there is the potential to flameout if the liquid fuel does not arrive quickly after the ramp-off of the gas fuel. Second, if liquid fuel does arrive in time, the control valve will be ramping up, bringing a larger amount of fuel into the combustion can than desired, which will cause a large spike in generated power, which could trip the unit due to over-temperature protection.
As combustion systems evolved to make use of multiple fuel nozzles in a combustor, the very low prefill values had to be raised to avoid possible damage to the fuel nozzles, this with the need for emergency transfers resulted in much higher prefill levels (approximately 6% to 8%). The potential side effects (over-temperature tripping, load shedding of the global fuel reference) are greatly increased if the prefill times are too long
Accordingly, there is a need to provide a method of prefilling liquid fuel system lines that can determine when the fuel lines are prefilled, thereby allowing fuel switchovers with greater reliability and smaller transients.